The present invention relates in general to medical imaging devices and techniques which use ultrasound. More particularly, the present invention pertains to hand-held, portable imaging probes and the structure of such probes which enable the automatic scanning of the transmitted ultrasound beam.
Medical imaging by the use of ultrasound had become a fairly well-known technique in the late 1960's and early 1970's. Early uses of ultrasonic techniques involved clinical diagnosis in the fields of obstetrics, urology and cardiology. More recently such techniques have become increasingly important in visualization of other body portions such as vascular applications and the scanning of breasts to detect tumors. Ultrasonic diagnostics has been shown to be capable of revealing, noninvasively, in pictorial fashion, tomographic cross sections of most internal human organs. In a number of cases, the use of ultrasound has made possible the diagnosis of disease (or the absence of), for example, cysts, tumors, or cancer in organs, in situations where other diagnostic methods such as radiography by themselves were found to be either inapplicable, insufficient, or unduly hazardous.
In those devices which utilize the pulse-echo method, pulses of ultrasonic energy are generated by a transducer, which has been properly energized by separate electrical circuitry. This ultrasonic energy generated by the transducer is focused and directed into the portion of the body to be imaged. When this focused beam (series of pulses) reaches an interface between two different materials or types of tissue, some of the transmitted ultrasound is reflected due to a characteristic impedance mismatch at the interface. In those devices employing a single transducer, it must operate in both the pulse and receive modes. In such an arrangement, after a pulse is transmitted, the transducer is placed in a receive mode in order to receive any reflected ultrasound and convert those received echoes into image-representative electrical signals. The time of arrival of the echo and its amplitude give information regarding the interface such as its depth into the body and the nature of the tissue. The image-representative electrical signals are able to be displayed in a number of different forms. The most common are referred to as an A-scan, B-scan, C-scan and M-scan. Each of these forms of display are well known and thoroughly described and analyzed in the ultrasound literature. Scan conversion circuitry is employed to receive and process the image-representative electrical signals prior to display in order to enable a diagnostically meaningful display.
A relatively popular form of data display for diagnostic imaging is the B-scan because the information is displayed in a manner similar to a conventional television. Radial or linear B scan is preferred for longitudinal and transverse cross sections. The B scan technique is two dimensional and provides a cross-sectional picture in the plane of the scan. The picture which results can also be recorded either photographically or on recording tape for retention and VCR playback.
Since ultrasonic imaging of the type mentioned is typically used as diagnostic tool, it is advantageous to be able to move the beam of ultrasound around the body so as to investigate different planes or slices. In arrangements with stationary transducers and ultrasound beams, it was found that predictability and uniformity of beam movement were important to the quality and diagnostic value of the resultant images. A result of this perceived importance on the predictability and uniformity of beam movement was to employ certain scanning arrangements whereby the beam of ultrasound could be mechanically or electrically moved in a predetermined and uniform manner. Although the probe which houses the transducer may still be moved manually over different portions of the body, an internal scanning technique which uniformly moves the beam in a precisely timed fashion, regardless of the probe placement on the body, is helpful in order to scan a larger area at each fixed position of the probe.
One such internal scanning technique is disclosed in U.S. Pat. No. 4,084,582 which issued Apr. 18, 1978 to Anant K. Nigam. Disclosed in this reference is a hand-held probe, which is fluid-filled and employs a fixed transducer and focusing lens. The scanning is performed by an oscillating reflector (mirror) which scans the generated beam of ultrasound pulses received from the transducer and reflected out through a flexible membrane covered window. In this particular approach, the scanning or oscillating mirror creates a sector scan and for distances deeper into the body the fanning out of the sector results in geometrical distortion when the image information is then later presented on a generally square or rectangular display. The problem which is encountered is that the density of information varies across the display and results in image information which is not as easily interpreted and may give rise to diagnostic error.
The present invention corrects this sector scan deficiency by the novel approach of using a lightweight focusing transducer which is moved back and forth linearly over the same axis thereby avoiding the sector scan problems. In a related embodiment a plurality of lightweight focusing transducers are used in order to create a real-time scan. A further advantage of the present invention is that it avoids the need for a focusing lens by a novel transducer design which provides the needed pulsing and receiving piezoelectric capabilities in a plano-concave form which as a result is self-focusing. By direct focus of the beam and the elimination of a reflective scanning mirror, concerns over total reflection from the mirror, the critical angle and material selection are obviated.